The manufacture of a pivot pin for a timepiece consists in performing bar turning operations on a hardenable steel bar to define various active surfaces (shoulder, projecting portion, pivots, etc.) and then in subjecting the bar-turned pin to heat treatments including at least one hardening operation to improve the hardness of the pin and one or more tempering operations to improve the roughness. The heat treatment operations are followed by an operation of rolling the pin pivots, which consists in polishing the pivots to the required dimensions. The rolling operation also improves the hardness and the roughness of the pivots. It will be noted that this rolling operation is very difficult or even impossible to achieve with materials having a low hardness, i.e. less than 600 HV.
The pivot pins, for example the balance staffs, conventionally used in mechanical timepiece movements are made in bar turning steel grades which are generally martensitic carbon steels including lead and manganese sulphides to improve their machinability. A known steel of this type, designated 20AP, is typically used for these applications.
This type of material has the advantage of being easy to machine, in particular of being suitable for bar turning and, after hardening and tempering, has superior mechanical properties which are very advantageous for making timepiece pivot pins. These steels have, in particular, superior wear resistance and hardness after heat treatment. Typically, the hardness of pin pivots made of 20AP steel can exceed 700 HV after heat treatment and rolling.
Although this type of material provides satisfactory mechanical properties for the timepiece applications described above, it has the drawback of being magnetic and able to disrupt the working of a watch after being subjected to a magnetic field, particularly when the material is used to make a balance staff cooperating with a balance spring made of ferromagnetic material. This phenomenon is well known to those skilled in the art and is for example described in the Bulletin Annuel Suisse de Chromométrie Vol. I, pages 52 to 74. It should also be noted that these martensitic steels are also corrosion sensitive.
Attempts have been made to try to overcome these drawbacks with austenitic stainless steels which have the peculiarity of being non-magnetic, namely paramagnetic or diamagnetic or antiferromagnetic. However, these austenitic steels have a crystallographic structure which means that they cannot be hardened or achieve hardnesses and thus wear resistances compatible with the requirements necessary for making timepiece pivot pins. One means of increasing the hardness of these steels is cold working; however this hardening operation cannot achieve hardnesses of more than 500 HV. Consequently, for parts which require high resistance to wear due to friction and pivots which have little or no risk of breakage or deformation, the use of this type of steel remains limited.
Another approach for attempting to overcome these drawbacks consists in depositing on the pivot pins hard layers of materials such as diamond-like-carbon (DLC). However, there have been observed significant risks of delamination of the hard layer and thus the formation of debris which can move around inside the watch movement and disrupt the operation of the timepiece, which is unsatisfactory.
Yet another approach has been envisaged for overcoming the drawbacks of austenitic stainless steels, namely the superficial hardening of the pivot pins by nitriding, carburizing or nitrocarburizing. However, these treatments are known to cause a significant loss of corrosion resistance because of the reaction of the nitrogen and/or carbon with the chromium in the steel and the formation of chromium nitride and/or chromium carbide causing localised depletion of the chromium matrix, which is detrimental to the desired timepiece application.